1. Field of the Invention
The present invention relates to a driving force distribution transmission for use with a vehicle with four-wheel drive and, more particularly, to a transmission which uses a hydraulic clutch to transmit rotary power from one drive shaft to the other drive shaft.
2. Description of the Prior Art
A conventional transmission of this kind is shown in FIG. 1, where a hydraulic clutch 3 is disposed between drive shafts 1 and 2 which are connected with the front wheels and the rear wheels, respectively, of a vehicle. A differential pump 4 produces a hydraulic pressure corresponding to the difference between the rotational speeds of the shafts 1 and 2. The oil delivered by the pump 4 is drained off to a tank 6 via a restrictor 5. The hydraulic pressure obtained ahead the restrictor 5 is caused to act on a piston 8 mounted in an oil chamber 7 formed in the clutch 3, pressing an inner plate 9b against an outer plate 9a inside the clutch 3. Thus, rotary power is transmitted from one of the two drive shafts to the other.
In this transmission, the transmitted torque T increases in proportion to the difference N between the rotational speeds of the two drive shafts, as indicated by the broken line A in FIG. 2. During turns, the front and rear wheels move along arcs having different radii and so the wheels are braked by the road surface. This phenomenon, hereinafter referred to as "braking", occurs in the region M in FIG. 2. In order to prevent the torque transfer characteristic curve A from passing through the region M, the torque T is made to increase at a low rate in proportion to the difference N. Accordingly, when the difference N slightly goes beyond the region M, the torque T is not large enough to permit the four-wheeled drive to operate. As a result, when the vehicle travels over road surfaces of small frictional coefficients, the driver cannot feel comfortable.